1. Field of the Invention
The present invention relates to a high-strength hot-dip galvanized steel sheet (including hot-dip galvannealed steel sheet) having a tensile strength of about 440-780 N/mm.sup.2 which is superior in formability and platability.
2. Description of the Related Art
The structural member of automobiles is made of steel sheet for absorption of impact energy. High-strength steel sheet for this purpose has been adopted for safety improvement and weight reduction (for improved fuel efficiency and environmental protection). Unfortunately, increase in strength leads to decrease in formability (or difficulties in press forming). Thus there has been a strong demand for steel sheets which meet requirements for both strength and formability.
Such steel sheets are disclosed in Japanese Patent Laid-open Nos. 26744/1992, 128320/1992, 128321/1992, 173945/1992, 331537/1993, 25537/1997, and 263883/1997. They include high-strength steel sheets having a high value of elongation due to deformation induced transformation of retained austenite, and they also include hot-dip galvanized steel sheets and hot-dip galvannealed steel sheets whose base metal has structure strength owing to the hard phase of martensite. They have been widely used as automotive steel sheets superior in corrosion resistance.
However, the prior art technology still has room for improvement. According to Japanese Patent Laid-open No. 26744/1992 mentioned above, the high-strength steel sheet with a high value of elongation is obtained by forming retained austenite in the hot-dip galvanizing line. It is poor in platability due to its high silicon content. In fact, this drawback has to be remedied by additional steps such as surface grinding and pre-plating, as illustrated in some examples. According to Japanese Patent Laid-open No. 331537/1993, the steel sheet has the composite structure of ferrite and martensite. This steel sheet is positively incorporated with Si and Mn for improvement in quench-hardenability. Si and Mn adversely affect platability, and this drawback has to be remedied by additional steps such as pre-plating. It is very difficult to make formability and platability compatible with each other. This difficulty has been overcome by carrying out additional steps such as surface grinding and pre-plating.
According to Japanese Patent Laid-open Nos. 128320/1992 and 128321/1992, the high-strength steel sheet with a high value of elongation is obtained by rapid cooling (which gives rise to a composite structure) in the hot-dip galvanizing line. The steel sheet is incorporated with silicon which concentrates carbon in austenite for improvement in quench-hardenability through martensitic transformation. (The amount of silicon is 0.17-0.20% in Examples.) Incorporation with an excess amount of silicon causes poor plating in alloying treatment (as mentioned in the documents). It is necessary to strictly control the amount of silicon (0.17-0.20%) so as to avoid surface defects (called bare-spot) due to wettability of molten zinc.
The steel sheet according to Japanese Patent Laid-open No. 173945/1992 has the composite structure of bainite-ferrite-martensite (with bainite being the major component). The disclosed technology requires the steel sheet to be incorporated with silicon for improvement in ferrite ductility by transfer of carbon into the austenitic phase. Incorporation with silicon produces the same adverse effect as mentioned above. The steel sheet incorporated with silicon is superior in local deformation such as bending but is poor in elongation because its structure is composed mainly of bainite which is a comparatively soft phase occurring by transformation at a low temperature.
Japanese Patent Laid-open Nos. 25537/1997 and 263883/1997 are concerned with the technology for a high-strength hot- and cold-rolled steel sheet which is composed of ferrite, martensite, tempered martensite, and lower bainite. This steel sheet needs a certain amount of titanium incorporated therein so that it has good resistance to pitting corrosion. Unfortunately, titanium forms coarse nitrides in steel, adversely affecting the local deforming performance. It also decreases the amount of carbon necessary to form carbides and to form the phase which undergoes transformation at a low temperature. Another disadvantage is an adverse effect on alloying at the time of plating, which hinders stable production.